Everything that drains to one place
Where Does a River's Water Come From?
Every drop of rain on Earth belongs to some river. Which one depends entirely on which side of a ridge it lands.
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Step 1 of 5
Water goes downhill, and that is all.
A raindrop takes the steepest way down, joins a trickle, then a stream, then a river. Nothing decides its route except the shape of the ground.
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Step 2 of 5
Ridges decide which river you belong to.
Along the top of a ridge is a line where rain landing a metre either side ends up in different rivers, and often different seas.
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Step 3 of 5
The branching pattern repeats at every size.
A basin looks the same whether you look at a whole continent or one hillside: a main channel with branches, each of which has branches of its own.
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Step 4 of 5
A basin is the honest unit for water.
What happens upstream arrives downstream — dirt, chemicals, floodwater. Managing water by basin rather than by border is the only arrangement that matches the physics.
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Step 5 of 5
Basins vary enormously in size.
The largest drains an area bigger than most continents' countries. The smallest is a gully behind a house. Both work in exactly the same way.
Some basins have no way out to the sea at all. Their rivers run into a lake or a salt flat and evaporate, which is why those lakes turn salty.
The short version
A river basin is all the land that drains into one river, divided from its neighbours by ridges rather than borders.
Try it yourself
Watch where water goes on a sloping path after rain. Trace it back uphill and you have mapped a tiny basin.